A polyurethane resin is produced by reacting a polyol with a polyisocyanate in the presence of a catalyst and, as the case requires, a blowing agent, a surfactant, a flame retardant, a cross-linking agent, etc. It is known to employ many metal-type compounds or tertiary amine compounds as catalysts for the production of polyurethane resins. Such catalysts are also industrially employed frequently as used alone or in combination.
In the production of a polyurethane foam using water or a low boiling point organic compound as a blowing agent, among these catalysts, a tertiary amine compound is particularly widely used, since the productivity and moldability will be thereby excellent. As such a tertiary amine compound, for example, conventional triethylenediamine, N,N,N′,N′-tetramethyl-1,6-hexanediamine, bis(2-dimethylaminoethyl) ether, N,N,N′,N″,N″-pentamethyldiethylenetriamine, N-methylmorpholine, N-ethylmorpholine, or N,N-dimethylethanolamine, may be mentioned (see e.g. Non-patent Document 1).
In the case of a metal type compound, the productivity and moldability tend to be poor, and therefore, in most cases, a tertiary amine is used in combination as a catalyst, and it is rare that a metal type compound is used alone.
Polyurethane resins, particularly flexible and semirigid polyurethane foams, are light in weight and excellent in elasticity, and therefore they are widely used for products such as vehicles, furniture, bedclothes, cushions, etc. Among flexible polyurethane foams, a flexible molded foam is used mainly in the automobile cushion field, and a flexible slab foam is used mainly in the field of furniture, bedclothes, cushions, etc. A semirigid foam is used in the field of head rests, arm rests, instrument panels, etc. for automobiles.
For the production of a urethane foam, the balance of two main reactions is important.
The first one is called a urethanization reaction (so-called “gelling reaction”), which is a reaction of a polyol and an organic polyisocyanate to form a urethane polymer and thus contributes to formation of the structure.
The second one is called a urea-forming reaction (so-called “blowing reaction”), which is a reaction of water and an organic polyisocyanate to form a urea polymer and thus contributes to foaming.
In order to obtain a urethane foam having a good structure, the gelling reaction and the blowing reaction must proceed simultaneously and at the most well-balanced rates. For example, if the blowing reaction proceeds fast, foam tends to collapse. On the other hand, if the gelling reaction proceeds fast, blowing by carbon dioxide gas tends to be prevented and cells tend to be closed, whereby shrinkage is likely to occur, or a high density foam is likely to be formed.
In recent years, in a method for producing a flexible urethane foam by mold forming, for the purpose of improving the productivity or reducing the costs for the foam, a demand for a high speed demolding system is increasing. In order to speed up the production cycle, it is required to increase the reactivity and shorten the process time, and, for example, it has been attempted to meet such a requirement by using a catalyst having a high activity or increasing the amount of such a catalyst. However, if the forming time is shortened by using a catalyst having a high activity or increasing the amount of such a catalyst, the gelling reaction tends to increase, and cells of foam tend to be closed and shrink. Thus, if the reactions are speeded up, the balance of the two reactions at the time of producing a urethane foam tends to be lost, and if the gelling reaction precedes, the above problem is likely to result.
As a method for solving such problems, it has been proposed to carry out the reaction in the presence of a salt of a tertiary amine and a carboxylic acid having a hydroxyl functionality, as a catalyst (see e.g. Patent Document 1).
However, in the method disclosed in Patent Document 1, the cell openness properties of the catalyst (a phenomenon wherein closed-cells formed at the time of forming a flexible polyurethane foam are broken to form open-cells immediately before curing (gelling)) are still not sufficient, and besides, the initial curing properties have been weakened by improving the cell openness properties, whereby finger marks (marks by fingers which attach to the foam at the time of demolding) are likely to attach. Thus, a further improvement of the catalyst has been desired.
The present inventors have already filed a patent application (see Patent Document 2) relating to a method for producing a polyurethane resin, wherein an amine compound containing at least one primary amino group and at least one tertiary amino group in its molecule, and a tertiary amine compound having at least two hydroxyalkyl groups in its molecule, are used in combination, as amine catalysts.
However, the method disclosed in Patent Document 2 is a method for producing, with good productivity and moldability, a polyurethane resin which is free from generation of a volatile amine and which undergoes less deterioration of the physical properties, and thus, the problems to be solved are different from the present invention.